Fix typo: s/performace/performance

[coldtobi]

a quick typo fix found by Debian's lintian tool.

[hzeller]

Thanks!
Can you also run src/create-manpage-inc.sh and add the resulting src/timg-manpage.inc to the PR ?
This is what emits the manpage with timg --help essentially as self-contained documentation.

(this really should be done as part of the build process, but I didn't get around yet to make that work with cmake...)

[hzeller]

Alright, will merge now, then update the inc file.